The influence of O(2) availability on the rate of protein synthesis, the levels of RNA and of adenylates, and the value of the energy charge in squash (Cucurbita maxima) cotyledons isolated from seeds germinated for 15 or 28 hours at different O(2) concentration (3% or 20% O(2)) has been investigated.The rate of protein synthesis is five times lower in cotyledons maintained in 3% O(2) than in those maintained in 20% O(2). Also net RNA synthesis is almost blocked in 3% O(2), while in 20% O(2) it proceeds almost linearly for 48 hours.The different RNA contents cannot explain the different rates of protein synthesis.The results of shift experiments (cotyledons shifted from 20% to 3% O(2) or vice versa) show that the rate of protein synthesis is strictly correlated with actual O(2) availability and is largely independent of the one in the previous period. O(2) controls the development of the adenylate pool and particularly the increase of ATP level. Thus, both the adenylate pool and the values of the energy charge ratio are lower in cotyledons grown in 3% than in 20% O(2).The shifts of O(2) availability induce rapid changes of ATP, ADP, and AMP levels and thus of the values of the energy charge, which are about 0.7 at 3% O(2) and higher than 0.8 at 20% O(2), independent of previous O(2) availability.The rate of protein synthesis appears to be largely independent of the levels of the single nucleotides and better correlated to the energy charge values.